If you're fitness-minded, chances are you've done it before. Come on, admit it: You've probably exercised in the morning thinking that you would burn more calories than if you exercised later in the day.

This is one case in which the early bird doesn't get the worm, because it doesn't matter. The number of calories you burn during a specific activity doesn't change according to whether it's morning, midday or evening. Rather, the number of calories you burn generally depends on your body composition, metabolism and food intake -- none of which are dependent on the time of day you choose to exercise.

Muscle consumes calories

Because muscle burns calories and body fat does not, the more lean muscle mass in your body and the less fat, the more calories you will burn during physical activity. Additionally, the more muscle mass you have, the more calories you will burn just sitting in a chair, cooking dinner or sleeping.

When people diet, particularly when they are not also exercising, they lose weight but often sacrifice muscle mass. This compounds the difficulty of losing weight and keeping it off. Severely restrictive diets can reduce your metabolic rate by up to 30 percent.

This is, in part, why strengthening exercises are so critical to losing and maintaining weight. Such exercises help you preserve muscle and bone, boosting your metabolism.

Three ways to burn calories

To better understand why the time of day you exercise does not affect the number of calories you burn, it is important to know how calories are used.

Basal metabolism: This is the energy you use to keep your body alive -- to maintain processes like the beating of your heart and functioning of other organs, brain activities and temperature regulation. It accounts for 50 to 75 percent of your daily caloric expenditure.

Use the formulas below to calculate your approximate basal metabolic rate (BMR). There are different formulas for men and women because men start out with more muscle mass than women and therefore burn more calories through their basal metabolism.

Women: 655 + (9.6 x W) + (1.7 x H) - (4.7 x A) = BMR

Men: 66 + (13.7 x W) + (5 x H) - (6.8 x A) = BMR

(W=weight in kilograms, H=height in centimeters, A=age in years)

• To convert kilometers to pounds: weight in pounds divided by 2.2

• To convert inches to centimeters: height in inches multiplied by 2.54

Digestion: This accounts for approximately 10 percent of the calories you use to process the food you consume. For instance, if you consume 2,000 calories a day, you burn 200 of them just by digesting the food. In effect, eating helps you burn calories in the same way exercise does, but to a lesser extent.

Physical activity: Various activities you perform throughout the day consume anywhere from 15 to 40 percent of the calories you burn. Physical activity is the most important component of caloric expenditure over which you have full control.

When people think of burning calories, they think of vigorous exercise, such as jogging or playing tennis. However, many people forget that all physical activity burns calories.

For example, people who take the elevator instead of the stairs -- but get up early to exercise in the hope of burning extra calories -- may be cheating themselves, because taking the stairs also burns calories.

Even activities as simple as getting up to change the television channel instead of using the remote control burn calories. Everyone is somewhat guilty of taking the easy route when it comes to getting somewhere or doing chores. But reminding yourself of the extra calories you burn during even the simplest tasks may help you to adopt a healthier lifestyle simply by taking advantage of your own mobility.

There are many ways to burn calories. The following table lists the number of calories a 150-pound person burns performing these activities for 20 minutes.

In the gym:

Step aerobics 168

Stationary bike (vigorous) 252

Stretching, hatha yoga 96

Stair-step machine 144

Low-impact aerobics 132

High-impact aerobics 168

Moderate calisthenics 108

Ski machine 228

Sports:

Playing basketball 192

Bowling 72

Disco dancing 132

Golf (carrying clubs) 132

Golf (using cart) 84

Cross-country hiking 144

Running (5.2 mph) 216

Swimming laps 240

At home:

Cooking 60

Washing the car 108

Reading 27

Sleeping 15

Watching TV 18

Painting the house 120

Source: Fitness Partner Connection

by Miriam Nelson, PhD
Created October, 2000